Clean and safe water is an indispensable resource essential for the survival of all species. Although 70% of the Earth is covered by water, only 2.5% of this is freshwater. Most of that water is unattainable, which leaves less than 1% of the Earth's freshwater for human consumption. According to the World Health Organization, one-sixth of the global population lacks access to clean drinking water. It would greatly benefit society, and save millions of lives, if we had safe, affordable, sustainable and easily deployable water purification techniques. Drinking water contamination can come from harmful microbiological pathogens, organic chemicals and heavy metals. Current water purification techniques include filtration, chemical means such as chlorination, ozonation, aeration, reverse osmosis, and ultraviolet (UV) radiation using UV-C lamps (operating at wavelength in the range 100-280 nm). Some of the above mentioned water purification techniques use lamps and require electricity to operate, some use slow physical separation processes, and others use chemicals that generate compounds leading to secondary pollution and even leave water with a bad smell and after taste. There is a pressing need for green, sustainable, easy to use, inexpensive and effective technologies for water purification.
Ultraviolet radiation from the sun (UV-A, wavelength of 315-400 nm) is a safe and cost-effective means to purify water infected by microbiological pathogens such as bacteria. When UV strikes the deoxyribonucleic acid (DNA) of bacteria, pyrimidine dimers or bonds are formed between adjacent thymine or cytosine base pairs. This inactivates the bacteria by preventing its DNA from replicating. Since the more harmful UV-B and UV-C radiation from the sun are blocked by the atmosphere, solar disinfection, commonly referred to as SODIS, that primarily uses UV-A radiation is very slow. In recent years photocatalysts such as TiO2 and ZnO, have been used to accelerate the photocatalytic SODIS process.
A photocatalyst is a substance, that when activated by light radiation (UV-A radiation and/or visible light, in the present invention), increases the rate of a reaction, without itself being consumed in the reaction. Anatase crystalline TiO2 and ZnO are photocatalysts that are activated by UV-A radiation from the sun. When UV-A radiation strikes these photocatalysts, electrons from the valence band are energized into the conduction band (e−) thereby leaving holes or positive charges (h+) in the valence band. Some of the electrons and holes may recombine, but most combine with oxygen and water to create reactive oxygen species such as super oxides (O2), hydroxyl radicals (.OH) and hydrogen peroxide (H2O2). These reactive species are responsible for the photo-killing of bacteria, reduction of heavy metals, and oxidation (and degradation) of organics into harmless species. The highly reactive oxygen species destroy pathogens by damaging cellular membranes, lipids, proteins and mitochondria. They also disrupt their DNA, alter their structure and prevent them from replicating.
One of the problems associated with the use of TiO2 for water purification, is the difficulty in recovering the TiO2 nanoparticles by filtration from the TiO2-water slurry, after the disinfection process. The slurry also impedes the transmission of UV, especially since TiO2 and ZnO are typically used in sunscreens to block UV radiation from the sun. In one of the field applications that use TiO2 assisted SODIS, TiO2 is mixed with perchloric acid and coated in the inner surface of plastic Polyethylene terephthalate (PET) water bottles. The bottles are filled with contaminated water and exposed to the sun, to be purified by photocatalysis. The primary drawback of this method is that the non-uniform TiO2 coatings block UV radiation which diminishes photocatalytic activity. Another drawback of this method is that the TiO2 coatings often wash-off after repeated use. Yet another limitation of the existing TiO2 enhanced SODIS method is its reduced efficiency when UV index is very low, since it uses UV-A radiation which comprises only 3% of the solar energy.